1. Field of the Invention
Generally, the present disclosure relates to the sophisticated integrated circuits including transistor elements comprising highly capacitive gate structures on the basis of a metal-containing electrode material and a high-k gate dielectric of increased permittivity compared to conventional gate dielectrics, such as silicon dioxide and silicon nitride.
2. Description of the Related Art
The fabrication of advanced integrated circuits, such as CPUs, storage devices, ASICs (application specific integrated circuits) and the like, requires the formation of a large number of circuit elements on a given chip area according to a specified circuit layout, wherein field effect transistors represent one important type of circuit element that substantially determines performance of the integrated circuits. Generally, a plurality of process technologies are currently practiced, wherein, for many types of complex circuitry, including field effect transistors, MOS technology is currently one of the most promising approaches due to the superior characteristics in view of operating speed and/or power consumption and/or cost efficiency. During the fabrication of complex integrated circuits using, for instance, MOS technology, millions of transistors, e.g., N-channel transistors and/or P-channel transistors, are formed on a substrate including a crystalline semiconductor layer. A field effect transistor, irrespective of whether an N-channel transistor or a P-channel transistor is considered, typically comprises so-called PN junctions that are formed by an interface of highly doped regions, referred to as drain and source regions, with a slightly doped or non-doped region, such as a channel region, disposed adjacent to the highly doped regions. In a field effect transistor, the conductivity of the channel region, i.e., the drive current capability of the conductive channel, is controlled by a gate electrode formed adjacent to the channel region and separated therefrom by a thin insulating layer. The conductivity of the channel region, upon formation of a conductive channel due to the application of an appropriate control voltage to the gate electrode, depends on the dopant concentration, the mobility of the charge carriers and, for a given extension of the channel region in the transistor width direction, on the distance between the source and drain regions, which is also referred to as channel length. Hence, in combination with the capability of rapidly creating a conductive channel below the insulating layer upon application of the control voltage to the gate electrode, the conductivity of the channel region substantially affects the performance of MOS transistors. Thus, as the speed of creating the channel, which depends on the conductivity of the gate electrode, and the channel resistivity substantially determine the transistor characteristics, the scaling of the channel length, and associated therewith the reduction of channel resistivity and increase of gate resistivity, is a dominant design criterion for accomplishing an increase in the operating speed of the integrated circuits.
Presently, the vast majority of integrated circuits are based on silicon due to substantially unlimited availability, the well-understood characteristics of silicon and related materials and processes and the experience gathered during the last 50 years. Therefore, silicon will likely remain the material of choice for future circuit generations designed for mass products. One reason for the dominant importance of silicon in fabricating semiconductor devices has been the superior characteristics of a silicon/silicon dioxide interface that allows reliable electrical insulation of different regions from each other. The silicon/silicon dioxide interface is stable at high temperatures and, thus, allows the performance of subsequent high temperature processes, as are required, for example, for anneal cycles to activate dopants and to cure crystal damage without sacrificing the electrical characteristics of the interface.
For the reasons pointed out above, in field effect transistors, silicon dioxide is preferably used as a gate insulation layer that separates the gate electrode, frequently comprised of polysilicon or other metal-containing materials, from the silicon channel region. In steadily improving device performance of field effect transistors, the length of the channel region has continuously been decreased to improve switching speed and drive current capability. Since the transistor performance is controlled by the voltage supplied to the gate electrode to invert the surface of the channel region to a sufficiently high charge density for providing the desired drive current for a given supply voltage, a certain degree of capacitive coupling, provided by the capacitor formed by the gate electrode, the channel region and the silicon dioxide disposed therebetween, has to be maintained. It turns out that decreasing the channel length requires an increased capacitive coupling to avoid the so-called short channel behavior during transistor operation. The short channel behavior may lead to an increased leakage current and to a dependence of the threshold voltage on the channel length. Aggressively scaled transistor devices with a relatively low supply voltage and thus reduced threshold voltage may suffer from an exponential increase of the leakage current while also requiring enhanced capacitive coupling of the gate electrode to the channel region. Thus, the thickness of the silicon dioxide layer has to be correspondingly decreased to provide the required capacitance between the gate and the channel region. For example, a channel length of approximately 0.08 μm may require a gate dielectric made of silicon dioxide as thin as approximately 1.2 nm. Although, generally, high speed transistor elements having an extremely short channel may preferably be used for high speed applications, whereas transistor elements with a longer channel may be used for less critical applications, such as storage transistor elements, the leakage current caused by direct tunneling of charge carriers through an ultra-thin silicon dioxide gate insulation layer may reach values for an oxide thickness in the range of 1-2 nm that may not be compatible with thermal design power requirements for performance driven circuits.
Therefore, replacing silicon dioxide as the material for gate insulation layers has been considered, particularly for extremely thin silicon dioxide gate layers. Possible alternative materials include materials that exhibit a significantly higher permittivity so that a physically greater thickness of a correspondingly formed gate insulation layer provides a capacitive coupling that would be obtained by an extremely thin silicon dioxide layer. Therefore, it has thus been suggested to replace silicon dioxide with high permittivity materials, such as tantalum oxide (Ta2O5) with a k of approximately 25, strontium titanium oxide (SrTiO3) having a k of approximately 150, hafnium oxide (HfO2), HfSiO, zirconium oxide (ZrO2) and the like.
Additionally, transistor performance may be increased by providing an appropriate conductive material for the gate electrode to replace the usually used polysilicon material, since polysilicon may suffer from charge carrier depletion at the vicinity of the interface to the gate dielectric, thereby reducing the effective capacitance between the channel region and the gate electrode. Thus, a gate stack has been suggested in which a high-k dielectric material provides enhanced capacitance based on the same thickness as a silicon dioxide layer, while additionally maintaining leakage currents at an acceptable level. On the other hand, the non-polysilicon material, such as titanium nitride and the like, may be formed so as to connect to the high-k dielectric material, thereby substantially avoiding the presence of a depletion zone. Since, typically, a low threshold voltage of the transistor, which represents the voltage at which a conductive channel forms in the channel region, is desired to obtain the high drive currents, commonly, the controllability of the respective channel requires pronounced lateral dopant profiles and dopant gradients, at least in the vicinity of the PN junctions. Therefore, so-called halo regions are usually formed by ion implantation in order to introduce a dopant species whose conductivity type corresponds to the conductivity type of the remaining channel and semiconductor region to “reinforce” the resulting PN junction dopant gradient after the formation of respective extension and deep drain and source regions. In this way, the threshold voltage of the transistor significantly determines the controllability of the channel, wherein a significant variance of the threshold voltage may be observed for reduced gate lengths. Hence, by providing an appropriate halo implantation region, the controllability of the channel may be enhanced, thereby also reducing the variance of the threshold voltage, which is also referred to as threshold roll-off, and also reducing significant variations of transistor performance with a variation in gate length.
On the other hand, the threshold voltage depends, in addition to the specific transistor configuration as described above, strongly on the work function of the gate electrode structure, which may be appropriately adjusted to the conductivity type and also to specific transistor characteristics, such as gate length and the like. The adaptation of the work function of the metal-containing electrode material may typically be accomplished by providing specific metal or metal alloys to obtain the required work function. It turns out, however, that presently the number of promising material candidates for adjusting the work function of sophisticated transistor elements may be moderately low, in particular when handling of these metals has to accomplished in very sophisticated manufacturing processes for fabricating semiconductor devices in accordance with volume production techniques. For example, titanium and aluminum, as well as any alloys thereof, may be used as gate electrode materials which, however, may specifically be adapted in material composition and the like to obtain the required work function and thus threshold adjustment. For example, in particular for sophisticated N-channel transistors, a moderately high work function of approximately 4.1 electron volts may be difficult to achieve since typically the work function has to be adjusted on the basis of an appropriate conductive barrier material that may have to be provided to guarantee integrity of the high-k dielectric material according to conventional process strategies. That is, according to a plurality of conventional process strategies, the high-k dielectric material may be provided in an early manufacturing stage and may thus pass through a plurality of process steps, such as a plurality of etch steps, high temperature treatments and the like, to complete the basic transistor configuration. Thereafter, in some of these approaches, a corresponding placeholder material, such as polysilicon, may be replaced by the desired metal electrode material, wherein, however, as previously explained, an appropriate material composition may have to be provided to obtain in combination with a conductive barrier layer, which may have to be maintained throughout the preceding process steps, the desired work function. For example, titanium nitride may frequently be used as a conductive barrier material which, however, may not readily allow a high work function as may be required for sophisticated N-channel transistors. On the other hand, avoiding the conductive barrier layer during the preceding process steps may be less than desirable due to a significant material erosion of the sensitive high-k dielectric material. Similarly, a removal of the conductive cap layer prior to the deposition of the work function metal may not represent a very promising approach due to a corresponding significant erosion of the high-k dielectric material.
In other conventional approaches, the high-k dielectric material and an appropriate work function metal for N-channel transistors may be provided in an early manufacturing stage and may then be patterned to obtain high-k gate electrode structures. In this case, however, a very complex manufacturing sequence may be required for maintaining the desired characteristics of the work function metal since frequently a significant drift may be observed after any high temperature processes. Additionally, the band gap of the channel material of P-channel transistors may be specifically adapted to the work function metal, which may frequently be accomplished on the basis of a silicon/germanium material, which may locally be provided within the channel region of the P-channel transistors. Consequently, a very complex process sequence prior to patterning the high-k metal gate electrodes may have to be performed which, in combination with a high probability of shifting the characteristics of the work function metal, may result in reduced performance of sophisticated semiconductor devices, which may render this approach, i.e., providing the high-k metal gate structure in an early manufacturing stage, a less attractive approach.
The present disclosure is directed to various methods and devices that may avoid, or at least reduce, the effects of one or more of the problems identified above.